This invention was made with government support under 5 RO1 DK34013 awarded by the National Institute of Diabetes and Digestive and Kidney Diseases. The government has certain rights in the invention.
The invention is directed to methods of targeting substances, including drugs, to specific types of liver cells, including hepatocytes.
An ability to specifically target pharmacological agents to hepatocytes, the major cell type in the liver, as well as to other cell types in the liver, would have important therapeutic implications in treating disorders and conditions affecting the liver. These disorders and conditions include viral infections such as hepatitis, viral-related conditions, and metabolic disorders, including disorders resulting from absent or defective genes or improper regulation of gene expression. Such a delivery mechanism could also be directed to the delivery of a specific factor that could regulate gene expression in hepatocytes. Such factors could possibly convert hepatocytes to perform some function that they do not normally carry out, such as secreting insulin.
The cells of the liver can be divided into two general classes: (1) hepatocytes or parenchymal cells; and (2) non-hepatocytes or non-parenchymal cells. The hepatocytes perform most of the metabolic functions of the liver. The non-hepatocytes include several cell types, including Kupffer cells. The Kupffer cells carry out phagocytosis.
Previous attempts to achieve such specific targeting have focused on using agents containing terminal galactose as a homing device for delivering drugs and other substances of interest to hepatocytes. There have been two basic approaches for using terminal galactose.
The first approach is direct conjugation of a functional group containing terminal galactose to the substance of interest. This procedure suffers from the need to develop a specific conjugation procedure for each substance of interest. Also, it can be difficult to perform the conjugation without impairing the pharmacological or physiological activity of the substance. Many conjugation reactions employ organic solvents, severe conditions such as high temperatures, or reagents such as glutaraldehyde that can inactivate labile substances such as polypeptides. Conjugation can also affect the immunogenicity of the substance, bringing about undesired allergic reactions or eliciting antibodies against it that can neutralize it.
The second approach has been the encapsulation of the substance of interest in a particulate carrier containing terminal galactose, such as liposomes or low density lipoproteins. This technology is used in U.S. Pat. No. 4,377,567 to Geho, U.S. Pat. No. 4,603,044 to Geho et al., Japanese Patent Publication No. 62201814, assigned to Daiichi Seiyaku, K. K. and Japanese Patent Publication No. 61112021 by Tomikawa et al.
This second approach has been disappointing. According to recent reports, a galactose receptor similar to that present on the surface of hepatocytes also exists on the surface of Kupffer cells. The ultimate cellular destination of carriers containing terminal galactose therefore depends on the number and positioning of the galactose groups on the surface of the carriers. The destination of the carrier is not readily predictable from the carrier used, the group conjugated, or the method of conjugation. Also, as with the method of direct conjugation to the substance, the conjugation of galactosyl groups to the surface of particulate carriers can result in adverse immunological reactions.
Other work has shown that there is a distribution of administered liposomes between parenchymal and non-parenchymal cells and that at high total dosages of liposomal lipid, at least 15-20 micrograms of liposomal lipid per gram of body weight, the liposomes are taken up predominantly by the parenchymal cells. This work includes F. Roerdink, J. Dijkstra, G. Hartman, B. Bolscher, & G. Scherphof, "The Involvement of Parenchymal, Kupffer and Endothelial Liver Cells in the Hepatic Uptake of Intravenously Injected Liposomes. Effects of Lanthanum and Gadolinium Salts," Biochim. Biophys. Acta 677, 79-89 (1981); H. H. Spanjer, H. Morselt, & G. L. Scherphof, "Lactosylceramide-Induced Stimulation of Liposome Uptake by Kupffer Cells in Vivo," Biochim. Biophys. Acta, 774, 49-55 (1984); and H. H. Spanjer, M. van Galen, F. H. Roerdink, J. Regts, & G. L. Scherphof, "Intrahepatic Distribution of Small Unilamellar Liposomes as a Function of Liposomal Lipid Composition," Biochim. Biophys. Acta 863, 224-230 (1986). However, the use of such excessively high doses of liposomal lipids results in a waste of liposomal lipid and substance to be carried, and may be undesirable for other reasons, such as not being suitable for those patients harmed by high dosages of lipids. This group includes patients with hyperlipidemias and certain other disorders.
Accordingly, there is a need for an improved method for selectively targeting substances such as drugs to classes of liver cells, especially hepatocytes. The method should avoid conjugation of galactose residues to either the substance to be carried or the carrier, prevent the occurrence of undesirable immunological reactions, be readily predictable in its effects, and avoid the administration of excess lipid.